In SharePoint 2007, you can have a column named "!".
I'm sure there are other characters you can use, as well, but this one surprised me. I suppose it is because I'm a C# developer…the "!" (or "bang") is an operator, not an identifier.
But in SharePoint (at least the 2007 version) it is a valid column name.
[Update: It appears that the column named "!" appears in SharePoint Designer as "_". I haven't tried including this column in any of my workflows, so I'm not sure how it would behave. If you have the opportunity to do so, please leave a note letting me know how it works for you. -sj]